Method of direct electroplating on a low conductivity material, and electroplated metal deposited therewith

ABSTRACT

A method of electroplating metal onto a low conductivity layer combines a potential or current reversal waveform with variation in the amplitude and duration of the applied potential or current pulse. The method includes, over time, varying the duration of the pulse and continuously decreasing the amplitude of both the cathodic and anodic portions of the waveform across the surface of the low conductivity layer as the deposition zone moves from the center of the surface of the low conductivity layer to the outside edge. By virtue of the ability to vary the amplitude and duration of the pulse, the method facilitates the filling of structures in the center of the low conductivity layer without overdepositing on the outside edge, thus ensuring a controlled deposition of material across the surface of the low conductivity layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to the field of electroplating.The invention relates more specifically to a method of electroplating,and a layer of electroplated material deposited therewith, that aresuitable for deposition on a low conductivity substrate material.

[0003] 2. Description of the Related Art

[0004] Various difficulties are associated with the electroplating ofmetals such as copper onto low conductivity barrier materials such as Taand W. In the context of semiconductor fabrication, the major problemassociated with plating on such barriers is that of achieving therequired adhesion and uniformity of the electroplated layer across thesurface of a wafer without the presence of fill defects.

[0005] Pulse plating, such as that disclosed in U.S. Pat. No. 5,972,192to Dubin et al., has historically been employed to platedifficult-to-plate materials or shapes. While conventional pulse platingtechniques can enable conformal plating across a surface under certaincircumstances, for the following reason these techniques are not alwayseffective for plating a resistive layer.

[0006] The electrical current is proportional to the voltage applied ata particular point (generally described by the Butler-Volmer Equation)as follows:

i=nFAk ^(o) [C _(o)(0, t)e ^(−αnf(E−E) ^(o′) ⁾ −C _(r)(0, t)e^(−(1−α)nf(E−E) ^(o′) ⁾].

[0007] Since the current is logarithmic with the applied potential(typically called overpotential E−E^(o′), where E is the applied voltageand E^(o′) is the formal potential defining the thermodynamicequilibrium point in a particular electrolyte), the potential needed tobe applied to the edge of an object in order to plate metal at thecenter of the object is well above that which is needed for plating atthe edge of the object near the contact point. The waveform createszones of excess plating toward the outside of the object, optimal growthrate in a finite region of the object, and no plating at the center ofthe object during the typical waveform (excluding the initialamplitudes). Therefore, with conventional techniques, the electroplateddeposits are typically excessively thick at the edge of an object, suchas a wafer, with minimal deposition at the center. Typicalelectroplating for wafers is accomplished with a dielectric materialplaced between the anode and cathode to modify the electric field.

[0008] In an attempt to overcome the non-uniform deposition, a highpulse amplitude technique has been employed. While the use of a highpulse amplitude may provide a more uniform deposit, it will also lead tofilling problems in the high aspect ratio features common tosemiconductor processes. To overcome such filling problems, the use of acurrent reversal waveform can be employed. The current reversal waveformcan deplate metal from the regions that are thicker, or deplate thethicker regions more quickly than the thin center portions, andtherefore increase the fill of high aspect ratio features. For example,U.S. Pat. No. 6,071,398 to Martin et al. discloses a method of pulseplating in which the ratio of peak reverse current density to peakforward current density is varied in periodic cycles. Martin, whichfocuses on achieving bottom up fill, discloses that the ratio is variedsequentially between first, second, and third values.

[0009] Electroplating a layer of metal on a layer of low conductivitymaterial, however, presents another obstacle. With the low conductivitymaterial, the IR drop across the surface of the low conductivitymaterial means that the filling is limited to a small portion of thesurface where the potential is defined by a narrow window.

[0010] Therefore, a need exists for a method of electroplating which notonly provides for the uniform filling of high aspect ratio features, butwhich also provides for the controlled deposition of a layer of desiredstructure and thickness across the entire surface of a low conductivitymaterial.

BRIEF SUMMARY OF THE INVENTION

[0011] The present invention provides a method of electroplating, and alayer of electroplated metal deposited therewith, that are suitable fordeposition on a layer of low conductivity material. More specifically,the present invention provides a method of electroplating and theresultant layer of electroplated metal that are characterized by thecontrolled deposition of a metal layer of desired structure andthickness across the entire surface of the low conductivity layer.

[0012] Accordingly, the present invention relates to a method ofelectroplating metal onto a low conductivity layer that combines apotential or current reversal waveform with variation in the amplitudeand duration of the applied potential or current pulse. The methodcomprises, over time, varying the duration of the pulse and continuouslydecreasing the amplitude of both the cathodic and anodic portions of thewaveform across the surface of the low conductivity layer as thedeposition zone moves from the center of the surface of the lowconductivity layer to the outside edge of the surface of the lowconductivity layer. The method thus advantageously uses the variablepotential associated with the IR differential from the center to theedge of the low conductivity layer.

[0013] By virtue of the ability to vary the amplitude and duration ofthe applied potential or current pulse, the method facilitates thefilling of structures in the center of the low conductivity layerwithout overdepositing on the outside edge, thus ensuring apredetermined profile of deposited material across the entire surface ofthe low conductivity layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Other features and advantages of the present invention willbecome more fully apparent from the following detailed description ofthe exemplary embodiments of the invention which are provided inconnection with the accompanying drawings.

[0015]FIG. 1 is a partial cross-sectional view of a wafer having a metallayer deposited in accordance with the present method of electroplating.

[0016]FIG. 2 is a plan view of the wafer depicted in FIG. 1.

[0017]FIG. 3 is a partial cross-sectional view of a chip produced fromthe wafer depicted in FIG. 1.

[0018]FIG. 4 illustrates the progressively decreasing waveformassociated with the present method of electroplating.

[0019] FIGS. 5A-R illustrate details of the method of depositing themetal layer depicted in FIG. 1.

[0020]FIG. 6 is a block diagram of a system for depositing the metallayer depicted in FIG. 1.

[0021]FIG. 7 is a block diagram of a system which employs the chip shownin FIG. 3.

[0022]FIG. 8 is a schematic diagram of the electroplating cell employedin the present method.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be understood from the exemplaryembodiments described herein.

[0024] The present invention relates to a method of electroplating metalonto a layer of low conductivity material. The method is particularlyuseful for the filling of structures in the center surface of the lowconductivity layer without overdepositing on the outside edge, thusensuring a controlled profile from deposition of the material across theentire surface of the low conductivity layer. The method provides astructure, such as for example, a semiconductor wafer or a chip producedtherefrom, comprising a substrate, a low conductivity layer, and auniform layer of electroplated metal deposited on the low conductivitylayer.

[0025]FIG. 1 is a partial cross-sectional view of a semiconductor wafer100 having a substrate 110 and a metal layer 130 deposited on a lowconductivity layer 120 in accordance with the present invention. The lowconductivity layer 120 may be barrier layer, such as for example, alayer of material selected from the group consisting of Ta, W, TaN/Ta,TaN, WN, W/WN, TaSiN, and TiSiN as is typically employed in asemiconductor chip to prevent alloying and copper migration duringmetallization thermal cycling. The low conductivity layer 120 mayinclude a high aspect ratio feature, such as a trench, that is an atleast partially filled damascene feature 121. Wafer 100 may also includefabricated circuit elements in and/or on substrate 110 and beneath thelow conductivity layer 120 which are not illustrated for purposes ofsimplicity.

[0026]FIG. 8 is a schematic view of the electroplating cell 800 employedin the present method, the details of the electroplating cell being wellknown in the art. Electroplating cell 800 includes generally a voltagesource 810, an anode 820, i.e., a donor metal (which may be inert anddecompose the electrolyte to balance electron transfer), and a cathode830 which includes the metal upon which the electrodeposition occurs.Cathode 830 (described above as wafer 100) initially includes thesubstrate 110 and the low conductivity layer 120. Cell 800 typicallyincludes a fluid inlet 840 and a fluid outlet 850 to provide for steadystate cell operation. The electroplating process may include rotation ofthe cathode 830 to facilitate the controlled deposition.

[0027] As indicated above in the Background section, electrical currentis proportional to the voltage applied at a particular point on thesurface of the electrode as generally described by the Butler-Volmerequation. Since the current is logarithmic with the applied potential,when a low conductivity material is employed as the material upon whichelectrodeposition is to occur, the potential needed at the edge of thewafer to plate metal at the center of the low conductivity layer is wellabove that which is needed at the edge of the wafer for plating at theedge of the low conductivity layer near the contact point. A typicalconventional waveform will creates zones of excess plating toward theoutside of the object, optimal growth rate in a finite region of theobject, and no plating at the center of the object (excluding theinitial amplitudes). Therefore, with conventional techniques, theelectroplated deposits are typically excessively thick at the edge of anobject, with minimal deposition at the center.

[0028] To overcome this drawback, a first embodiment of the presentmethod utilizes the combination of two elements: a potential reversalwaveform, and variation in the amplitude and duration of the appliedpotential pulse. The method, therefore, advantageously uses the variablepotential associated with the IR differential from the center to theedge of the low conductivity layer. The first element of the methodincludes applying a pulsed periodic reverse potential comprising asequential forward to reverse, reverse to forward, continuouslyrepeating pulsing sequence across the electrodes of the electroplatingcell. The pulsing sequence utilizes a potential reversal waveform havinga peak reverse potential density and a peak forward potential density.

[0029] The second element of the method includes varying the amplitudeand duration of the voltage applied to the electroplating cell. Morespecifically, the amplitude-varying feature employs variable amplitudeprogrammed decay. FIG. 4 illustrates the progressively decreasingwaveform associated with the varied amplitude. The amplitude of thecathodic and anodic portions of the potential reversal waveform iscontinuously decreased with increasing time, i.e., decreased as theelectrodeposition proceeds from the time of its initiation to the timeof its conclusion. The initial high pulse waveform (both plating anddeplating) focuses the plating in the center 122 of the low conductivitylayer 120. With increasing time, the deposition proceeds outward acrossthe surface of the low conductivity layer 120, i.e., from the center 122of the low conductivity layer 120 to the edge 123 of the lowconductivity layer 120 in order to deposit the metal layer 130. The unitfor the time scale depicted in FIG. 4 depends on the frequency of thepotential pulse. For example, a waveform at 1 Hz will result in a timescale on FIG. 4 of around 100 seconds. At 1 KHz, the time scale would beonly about 1 second on FIG. 4.

[0030] As indicated above, the second element of the method alsoincludes varying the duration of the applied potential pulse. Theduration of the pulsed voltage may be increased, decreased, or increasedand decreased, depending upon the particular application. The durationfor each portion of the waveform may be independently controlled. A restperiod, shown in FIG. 4, may or may not be needed, depending on theresistance of the material and/or the location of the electroplating onthe wafer.

[0031] The amplitude of the cathodic and anodic portions of thepotential reversal waveform is continuously decreased, and the durationof the pulse is varied, in a manner that can provide a metal layer 130having a desired structure and thickness across the surface of the lowconductivity layer 120. As illustrated in FIG. 2, the deposition ofmetal layer 130 proceeds from the center 122 of the low conductivitylayer 120 to the outside edge 123 of the low conductivity layer 120 inan approximately concentric ring configuration. In a typical embodiment,the metal layer 130 has a uniform thickness, from the center 122 of thelow conductivity layer 120 to the outside edge 123 of the lowconductivity layer 120, of from about 50 angstroms to about 3000angstroms. Thus, by virtue of the ability to vary the amplitude andduration in a specific combination, a predetermined profile of depositedmetal layer 130 across the surface of low conductivity layer 120 can beachieved.

[0032] Details of the method of depositing a metal layer 538 (analogousto the metal layer 130 depicted in FIG. 1) are illustrated in FIGS.5A-R. FIGS. 5A-R thus provide a “snapshot” view at different times ofthe ring-type plating that is characteristic of the present method.FIGS. 5A, 5C, 5E, 5G, 5I, 5K, 5M, 5O, and 5Q provide a partialcross-sectional view of the wafer 100 as the electrodeposition proceeds.FIGS. 5B, 5D, 5F, 5H, 5J, 5L, 5N, 5P, and 5R provide a graphicalillustration of the cation concentration versus distance from thesurface of low conductivity layer 120 during the stages of depositionrepresented by FIGS. 5A, 5C, 5E, 5G, 5I, 5K, 5M, 5O, and 5Qrespectively. In FIGS. 5B, 5D, 5F, 5H, 5J, 5L, 5N, 5P, and 5R, the term“bulk concentration” refers to the nominal concentration of theelectroplating solution, i.e., the concentration of copper cationssufficiently far enough away from the electrode surface that theconcentration is fixed.

[0033] FIGS. 5A-F represent the initial stage of the electrodepositionprocess, i.e., that point at which the process focuses the plating inthe center 122 of the low conductivity layer 120. FIGS. 5A and 5Bcorrespond to the largest amplitude for the forward voltage pulse. FIGS.5C and 5D correspond to the largest forward and reverse voltageamplitudes during the deplating pulse. FIGS. 5E and 5F correspond to thelargest amplitude of the forward pulse during the rest period.

[0034] FIGS. 5G-L represent an intermediate stage of theelectrodeposition process, i.e., that point at which the plating isfocused at a point intermediate between the center 122 and the edge 123of the low conductivity layer 120. FIGS. 5G and 5H correspond to amid-range amplitude for the forward voltage pulse. FIGS. 5I and 5Jcorrespond to mid-range forward and reverse voltage amplitudes duringthe deplating pulse. FIGS. 5K and 5L correspond to the mid-range forwardand reverse voltage amplitudes during the rest period.

[0035] FIGS. 5M-R represent the final stage of the electrodepositionprocess, i.e., that point at which the plating is focused at a pointnear the edge 123 of the low conductivity layer 120. FIGS. 5M and 5Ncorrespond to the smallest amplitude for the forward voltage pulse.FIGS. 5O and 5P correspond to the end of the forward and reverse pulsesduring the deplating step. FIGS. 5Q and 5R correspond to the end of theforward and reverse pulses during the rest period.

[0036] As is evident from FIGS. 5 A-R, the initial high pulse waveform(both plating and deplating) focuses the plating in the center 122 ofthe low conductivity layer 120. The amplitude of both the cathodic andanodic portion of the waveform, however, is then continuously decreased,and the duration of the pulse varied, in a manner that will provide apredetermined deposition profile across the surface of the lowconductivity layer 120. With time, therefore, the deposition zoneproceeds to the outside edge in a ring-type configuration (see also FIG.2). The relaxation of the surface from the rest period with a pulse-typewave provides for more conformal plating at the beginning of the platingprocess. The changing pulse amplitude and duration facilitates controlof the uniformity of deposition and the final fill of structures such asdamascene feature 121. The changing of the amplitude of the forward andreverse cycle facilitates filling structures in the center 122 of thelow conductivity layer 120 without overdepositing on the outside edge123.

[0037] Furthermore, since a pulse reverse type waveform is incorporatedinto the total waveform, enhanced fill from the bottom up occurs. Thus,for example, the method facilitates the fill of a trench to providefeatures such as damascene feature 121. The rest period keeps theelectrodeposition process at a non-diffusion-limited regime for platingand may allow the organic additives to redistribute onto the appropriatesites.

[0038] The frequency of the voltage pulse is typically from about 1 Hzto about 100 KHz. Use of a higher frequency will typically not allowsignificant movement of the metal atoms. The range of applied potentialamplitudes that is employed depends upon certain variables associatedwith the electrodeposition process. For example, potentials forelectrochemical cells are typically compared using a reference electrodein order to compensate for the resistance drop associated with the celldesign. There is also a resistance drop associated with the lowconductivity layer 120, which is a function of both the low conductivitymaterial and its thickness. If the interface of the electrodes resultsin a drop of more than about 2 volts, the electrolyte will break downwith the evolution of hydrogen or oxygen. Therefore, in a typicalembodiment of the present method, the amplitude can range from severaltenths of a volt for very conductive films with good cell designs, toabout 5 volts. The potential drop across the surface of the lowconductivity layer 120 is dependent upon the thickness of the layer 120.For example, when layer 120 is tungsten, the potential drop is from 3-5ohms/sq.

[0039] In an optional embodiment, the method may further includepretreatment of the low conductivity layer 120 prior to initiating theelectroplating process. For example, the pretreatment could be employedto clean the surface of the low conductivity layer 120, such as forexample, by using ammonium hydroxide or hydrofluoric acid to removeoxide from a tungsten barrier layer.

[0040] Thus, the present method takes advantage not only of therelaxation (i.e., the mass transport of copper to the surface) of themetal layer 130, but of the variable potential across the surface of thelayer 120 of low conductivity material created by the IR drop of the lowconductivity layer. Combining these features allows one to control boththe deposition location and the deposition quantity of the metal layer130.

[0041] A second embodiment of the present method utilizes thecombination of two elements: a current reversal waveform, and variationin the amplitude and duration of the applied current pulse. The firstelement of the second embodiment of the method includes applying apulsed periodic reverse current comprising a sequential forward toreverse, reverse to forward, continuously repeating pulsing sequenceacross the electrodes of the electroplating cell. The pulsing sequenceutilizes a current reversal waveform having a peak reverse currentdensity and a peak forward current density. The second element of thesecond embodiment of the method includes varying the amplitude andduration of the current applied to the electroplating cell.

[0042] The preferred chemistry of the electroplating solution is acomplexed basic bath in which the potential obtained during the cathodicportion of the waveform is capable of reducing oxide on the surface ofthe barrier without significant metal plating. The electrodeposition ofmetal layer 130 is effected in an electroplating cell utilizing acompleted basic solution which comprises an aqueous basic metalelectrolyte. Depending upon the particular structure desired, the metalthat is deposited according to the present method may be most any metalthat can be electrodeposited from aqueous chemistries, such as, forexample, Cu, Ni, Au, Cr, Ag, Pt, and Ir. If the electrodeposited metalis copper, for example, the copper electrolyte can be cupric sulfate.

[0043] In one embodiment of the method for the electrodeposition ofcopper, the electroplating solution (i.e., “bath”) includes a complexedbasic solution comprising cupric sulfate and a solution ofethylenediamine tetraacetic acid (“EDTA”) and tetramethylammoniumhydroxide (“TMAH”). In this embodiment, the bath comprises a solution offrom about 1 to about 10 g/l of CuSO₄, typically from about 5 to about 6g/l of CuSO₄, in from about 35 to about 45 g/l of EDTA, typically fromabout 40 to about 43 g/l of EDTA. The bath also typically comprises fromabout 1 to about 5 ml (per liter of electroplating solution) of asurfactant, such as TRITON X-100 (commercially available from UnionCarbide), and from about 20 to about 100 ml of 25% TMAH. In an optionalembodiment, the electroplating solution may comprise a citric acidsolution or other metal complexing acid bath.

[0044]FIG. 3 is a partial cross-sectional view of a chip 200 producedfrom the wafer depicted in FIG. 1. Chip 200 includes a substrate 210 anda metal layer 230 deposited on a low conductivity layer 220. Chip 200may then be incorporated in any fabricated semiconductor device,including various processor system components, such as for example, acentral processing unit (“CPU”) or in any of the various types of memorydevices, such as for example, RAM, ROM, and others. It may also be usedin any type of integrated circuit controller for a floppy disk, a harddisk, a ZIP, or a CD-ROM disk.

[0045]FIG. 6 is a block diagram of a system 600 for depositing the metallayer depicted in FIG. 1. The system 600 comprises an electroplatingcell 610, and a processor system 620. The processor system 620 iscapable of operating the electroplating cell 610 so as to provide alayer of electroplated metal 130 having uniform structure and thicknessacross the surface of the low conductivity layer 120. The variableamplitude programmed decay, and the variation in pulse duration, areeffected through control of the power supply associated with theelectroplating cell 610. For example, processor system 620 and theassociated software may be employed to control the power supply bysending a digital or analog signal to effect a particular rate of decay.The decay rate can be determined by any of various mathematicalfunctions, such as, for example, a linear or exponential decay function,or another function capable of effecting a particular decay rate. Anyparticular decay rate is dependent upon the material of metal layer 130,the thickness of the deposited metal layer 130, the material andthickness of the low conductivity layer 120, and the chemistry of theelectroplating solution.

[0046]FIG. 7 is a block diagram of a system 700 utilizing a chip 200(see FIG. 3) comprising a layer of metal deposited in accordance withthe present invention. System 700 typically comprises a CPU 710. Thesystem 700 may be a computer system, a process control system, or anyother system employing a processor and associated memory, and may employone or more buses and/or bridges which allow the CPU 710 to internallycommunicate with I/O devices 720, 730, random access memory (RAM)devices and read-only memory (ROM) devices 740, and peripheral devicessuch as a floppy disk drive 750 and a compact disk CD-ROM drive 760 thatalso communicate with CPU 710 over the bus 770, as is well known in theart. As discussed above with respect to chip 200, any of the CPU 710,the memory devices, and controller elements of other illustratedelectrical components may include a chip 200 having a layer ofelectrodeposited metal 230 deposited in accordance with the claimedinvention.

[0047] The present invention, therefore, provides a method ofelectroplating, and a layer of electroplated metal deposited therewith,that are suitable for deposition on a layer of low conductivitymaterial. By virtue of the ability to vary the amplitude and duration ofthe applied potential or current pulse, the method facilitates thefilling of structures in the center of the low conductivity layerwithout overdepositing on the outside edge, thus ensuring a controlleddeposition of material across the entire surface of the low conductivitylayer.

[0048] Although the invention has been described and illustrated asbeing suitable for use in semiconductor applications, for example,processor systems and memory devices, the invention is not limited tothese embodiments. Rather, the invention could be employed in anyservice requiring controlled uniformity of an electrodeposited metalonto a layer of low conductivity material.

[0049] Accordingly, the above description and accompanying drawings areonly illustrative of exemplary embodiments that can achieve the featuresand advantages of the present invention. It is not intended that theinvention be limited to the embodiments shown and described in detailherein. The invention is limited only by the scope of the followingclaims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method of electroplating metal onto a layerof low conductivity material, said method comprising: applying a pulsedperiodic reverse potential comprising a sequential forward to reverse,reverse to forward, continuously repeating pulsing sequence across theelectrodes of an electroplating cell utilizing a potential reversalwaveform having a peak forward potential density and a peak reversepotential density; and varying the amplitude and duration of the appliedpotential of the cathodic and anodic portions of the potential reversalwaveform to deposit a layer of electroplated metal.
 2. The method ofclaim 1, wherein said amplitude of the applied potential is continuouslydecreased over time at a rate capable of depositing said layer ofelectroplated metal with a desired uniform structure and thicknessacross the surface of said low conductivity layer.
 3. The method ofclaim 1, wherein said deposition proceeds from the center of the surfaceof said low conductivity layer to the edge of the surface of said lowconductivity layer.
 4. The method of claim 3, wherein the deposition hasan approximately concentric ring configuration.
 5. The method of claim1, wherein said duration of the applied potential is decreased.
 6. Themethod of claim 1, wherein said duration of the applied potential isincreased.
 7. The method of claim 2, wherein said uniform thickness isfrom about 50 angstroms to about 3000 angstroms.
 8. The method of claim1, wherein the frequency of said potential pulse is from about 1 Hz toabout 100 KHz.
 9. The method of claim 1, wherein said amplitude is fromabout 0.5 V to about 5 V.
 10. The method of claim 1, wherein saidelectroplating is effected in a complexed basic solution.
 11. The methodof claim 10, wherein said completed basic solution comprises an aqueousbasic metal electrolyte.
 12. The method of claim 11, wherein said metalis selected from the group consisting of Cu, Ni, Au, Cr, Ag, Pt, and Ir.13. The method of claim 12, wherein said metal is Cu.
 14. The method ofclaim 11, wherein said aqueous basic metal electrolyte is cupricsulfate.
 15. The method of claim 11, wherein said complexed basicsolution comprises a solution of from about 1 to about 10 g/l of cupricsulfate.
 16. The method of claim 15, wherein said complexed basicsolution comprises a solution of from about 5 to about 6 g/l of cupricsulfate.
 17. The method of claim 11, wherein said complexed basicsolution comprises a solution of EDTA and TMAH.
 18. The method of claim17, wherein said EDTA and TMAH solution comprises from about 35 to about45 g/l of EDTA.
 19. The method of claim 18, wherein said EDTA and TMAHsolution comprises from about 40 to about 43 g/l of EDTA.
 20. The methodof claim 17, wherein said EDTA and TMAH solution comprises from about 20to about 100 ml of 25% TA.
 21. The method of claim 11, wherein saidcomplexed basic solution further comprises a surfactant.
 22. The methodof claim 10, wherein said complexed basic solution comprises citricacid.
 23. The method of claim 1, wherein said layer of low conductivitymaterial is a barrier layer.
 24. The method of claim 1, wherein said lowconductivity material is selected from the group consisting of Ta, W,TaN/Ta, TaN, WN, W/WN, TaSiN, and TiSiN.
 25. The method of claim 1,further comprising pretreating said low conductivity layer beforeinitiating said electroplating.
 26. A system for electroplating metalonto a layer of low conductivity material, said system comprising: anelectroplating cell; and a processor system for operating saidelectroplating cell by applying a pulsed periodic reverse potentialcomprising a sequential forward to reverse, reverse to forward,continuously repeating pulsing sequence across the electrodes of saidcell utilizing a potential reversal waveform having a peak forwardpotential density and a peak reverse potential density; and varying theamplitude and duration of the applied potential of the cathodic andanodic portions of the potential reversal waveform to deposit a layer ofelectroplated metal.
 27. A semiconductor wafer comprising: a substrate;a layer of low conductivity material disposed on said substrate; and alayer of electroplated metal deposited on said low conductivity layer byapplying a pulsed periodic reverse potential comprising a sequentialforward to reverse, reverse to forward, continuously repeating pulsingsequence across the electrodes of an electroplating cell utilizing apotential reversal waveform having a peak forward potential density anda peak reverse potential density; and varying the amplitude and durationof the applied potential of the cathodic and anodic portions of thepotential reversal waveform.
 28. A semiconductor chip comprising: asubstrate; a layer of low conductivity material disposed on saidsubstrate; and a layer of electroplated metal deposited on said lowconductivity layer by applying a pulsed periodic reverse potentialcomprising a sequential forward to reverse, reverse to forward,continuously repeating pulsing sequence across the electrodes of anelectroplating cell utilizing a potential reversal waveform having apeak forward potential density and a peak reverse potential density; andvarying the amplitude and duration of the applied potential of thecathodic and anodic portions of the potential reversal waveform.
 29. Amemory device comprising a semiconductor chip, said chip comprising: asubstrate; a layer of low conductivity material disposed on saidsubstrate; and a layer of electroplated metal deposited on said lowconductivity layer by applying a pulsed periodic reverse potentialcomprising a sequential forward to reverse, reverse to forward,continuously repeating pulsing sequence across the electrodes of anelectroplating cell utilizing a potential reversal waveform having apeak forward potential density and a peak reverse potential density; andvarying the amplitude and duration of the applied potential of thecathodic and anodic portions of the potential reversal waveform.
 30. Asystem comprising: a processor; and a memory device coupled to saidprocessor, at least one of said processor and said memory devicecomprising a semiconductor chip, said chip comprising: a substrate; alayer of low conductivity material disposed on said substrate; and alayer of electroplated metal deposited on said low conductivity layer byapplying a pulsed periodic reverse potential comprising a sequentialforward to reverse, reverse to forward, continuously repeating pulsingsequence across the electrodes of an electroplating cell utilizing apotential reversal waveform having a peak forward potential density anda peak reverse potential density; and varying the amplitude and durationof the applied potential of the cathodic and anodic portions of thepotential reversal waveform.
 31. A method of electroplating metal onto alayer of low conductivity material, said method comprising: applying apulsed periodic reverse current comprising a sequential forward toreverse, reverse to forward, continuously repeating pulsing sequenceacross the electrodes of an electroplating cell utilizing a currentreversal waveform having a peak forward current density and a peakreverse current density; and varying the amplitude and duration of theapplied current of the cathodic and anodic portions of the currentreversal waveform to deposit a layer of electroplated metal.
 32. Amethod of electroplating metal onto a layer of low conductivitymaterial, said method comprising: applying a pulsed periodic reversepotential waveform comprising a sequential forward to reverse, reverseto forward, continuously repeating pulsing potential across theelectrodes of an electroplating cell utilizing a potential waveformhaving a peak forward potential and a peak reverse potential; andvarying the amplitude and duration of the applied potential of thecathodic and anodic portions of the potential waveform to deposit alayer of electroplated metal.
 33. A method of electroplating metal ontoa layer of low conductivity material, said method comprising: applying apulsed periodic reverse current controlled waveform comprising asequential forward to reverse, reverse to forward, continuouslyrepeating pulsing current across the electrodes of an electroplatingcell utilizing a current controlled waveform having a peak forwardcurrent and a peak reverse current; and varying the amplitude andduration of the applied current of the cathodic and anodic portions ofthe current waveform to deposit a layer of electroplated metal.